Women artists are staggeringly under-represented in the world's major galleries. Can these masked activists put things right?

· The Guerrilla Girls will be at Tate Modern on Saturday from 3-4.30pm as part of the UBS Openings: Saturday Live season. Details: 020-7887 8888 or Tate.org/modern

I have never walked into a lecture hall and not been bored at the very thought of it, until last weekend. The Guerrilla Girls are, I have to admit, wearing gorilla masks. One has a big, laughy face, and the other has a more contemplative expression; these traits are echoed, I think, by the personalities of the women inside. Of course it enhances any gathering when some people are dressed as animals, but that is not why they're engaging. They're engaging because they're fast-talking and reactive and witty, but most of all because they are making a very clear, unarguable point - that women are under-represented in the art world.

The Guerrilla Girls have been pointing this out since 1985, when they picketed the Museum of Modern Art in New York. At the time it was running a show that purported to be the definitive survey of contemporary painting and sculpture, and only 13 of the 169 artists featured were women. But nobody really paid any attention to them, which is why they put on the masks - and they haven't taken them off since (except, perhaps, for baths).

To cover the basics of guerrilla: there are several of them but they are all anonymous, and, when asked for their names, they will assume the identity of a dead female artist. Mine were Frida Kahlo and Kathe Kollitz.
Before I met them, I'll tell you what the guerrilla concept reminded me of - Bolsheviks, before the revolution, when they didn't have to worry about governing, and only had to say: "Peace! Bread! Land!" You're never going to get any argument there; everybody likes bread, peace and land. And surely everybody, broadly, would agree that women should be better represented by the art establishment - particularly when they aren't exactly in a tiny minority in the community of practising artists. But it turns out that the French for one don't agree: there was an exhibition called Dionysiac in Paris at the Pompidou Centre last year with no women artists, which prompted critics and curators rather unabashedly to speculate as to whether women were capable of creativity at all. "Curators were bragging," say the Guerrilla Girls, "they were bragging about how only men are doing work that's worthwhile!" Even apart from the cheese-eating misogyny monkeys, everybody, globally, is moving incredibly slowly on this; women artists working today still earn the most shaming fraction of what men earn.

The Guerrilla Girls, rather depressingly I think, seem to take this lack of progress with a shrug, averring that chauvinism is always with us - it simply takes different forms for different generations. Part of this, no doubt, is a reluctance to make vast claims for their own influence. "We've had a number of periods of self-criticism," the mirthless gorilla says at one point. "We're not perfect. We're really funny, but we're not perfect." But part of it, also, is that America is getting more, not less, conservative, and this is clearly having an impact on the art world. "Let's just say there's probably no one in the Bush administration who subscribes to our point of view," says the cheerful gorilla.

The Guerrilla Girls are in London this week to give a series of lectures, and also to visit the room now given over to their polemical posters as part of Tate Modern's rehang. One of their first and possibly most famous was a 1989 rendering of Ingres's Odalisque wearing a gorilla's head, and the strapline: "Do Women Have to be Naked to Get into the Met Museum?" But before that, in 1986, they did a bald, black on white, text-only poster that said: "IT'S EVEN WORSE IN EUROPE." "We didn't even have to explain it, we didn't have to do anything. Everybody knew what it meant!" says the laughing gorilla.

They say it's amazing how little has changed: in 2005, they conducted some fresh research into European collections, and found that, even where museums owned significant amounts of work by female artists, it was all in the basement. To coincide with last September's Venice Biennale, they released the following statistics: of 1,238 artworks exhibited by the major Venice spaces, fewer than 40 are by women. They believe the city's Museo Correr actively does women a disservice by collecting their work, since they have 15 pieces by women in their collection and not one of them is on show. One of the Guerrilla Girls' Ten Commandments is: "Thou shalt provide lavish funerals for women and artists of colour who thou planneth to exhibit only after their death." The swizz of a posthumous fanfare must be matched only by the insult of seeing your work ... well, never seeing your work, and knowing no one else will see it, either.

Amusingly, the Guerrilla Girls are not that sold on Tate Modern, or its rehang. "We should be having an installation in the Turbine Hall about art in Britain - why are we stuck in this little room?" says the non-laughing one, and the laugher tries to smooth things over with: "But on the other hand, we're glad to have this space, if only to encourage other people to question the Tate." There's a pause; I get the impression she's fighting her natural tact. "To be honest, 16% [women] in the rehang is not fantastic."

I raised this with Tate curator Frances Morris, who is just about the most cautious woman I've ever spoken to. "It's a really important issue to raise, and keep raising, for institutions and the public ... It seems like a very simple message, but in fact it's a very complicated one." I asked her about ethnic representation, which is another hot-button issue for the Guerrilla Girls. "I ... I think that's a difficult thing to answer. There is much more of a sense of global balance ... I don't think I want to talk about diversity." Honestly, you'd get more straight talking out of Alastair Campbell.

The Guerrilla Girls say that tokenism is "rampant in the American art world. Suddenly, in the early 90s, the art world became fascinated with multi-culturalism. You could have written their small ads - they would have read, 'Curator seeks artist of colour for one show every five years. Must have shown in every other museum.'" I ask them if they think this has had an impact on the kind of work that finds its way into galleries - whether there's a vogue for women artists who are intensely focused on their sex so that gallerists can squeeze maximum goodwill out of the gesture. "That's something for other people to tell us," laughing gorilla says evenly. "We would encourage your readers to go to galleries [that] under-represent women and report back on whether that has an effect on the kind of art they're showing."

Interestingly, for the first time in 20 years, the Guerrilla Girls are concentrating their energies on Hollywood and the film industry, "which is where the art world was 25 years ago: it's shocking". Earlier this year, around the time of the Academy awards, they put up billboard posters in Los Angeles pointing out that no woman has ever won an Oscar for directing a film, and that only three have even been nominated (Sophia Coppola, Jane Campion, Lina Wertmuller in 1976).

Someone comes up to us and says: "How do you get to be a guerrilla girl?" Very charmingly they say that, well, there are tons of them, and they're not canvassing for more members. "But you know," says stern gorilla, "you could set up on your own. Find something that makes you really angry and aestheticise it. I don't enjoy wearing a gorilla mask, but that's how I'm taken seriously. You don't have to be gorillas. There's always room in the world for more masked feminist avengers"

'They're as strong as men'

"I think things are on an up at the moment. Take the Venice Biennale - it's the first time in more than 100 years women have even curated the show. There is still a discrepancy in prices at the high end, but that will change as more women buy more art."

Stella Vine, artist

"I don't think women are under-represented in the art world. There are plenty of good women artists. But I do think they are underpriced."

Sadie Coles, director of the Sadie Coles gallery

"Nowadays women can own and control their own galleries. Five out of the 12 artists I represent are women. Women have changed: they are more independent, more ambitious. The role of women and men is almost equal, and that's reflected in the arts scene."

Virginia Damtsa, director of the Riflemaker gallery

"I'm proud of the Whitechapel's history of pioneering the work of female artists. I curated the first MaxMara art prize for women this year, and it was fantastic to dedicate a prize to the work of so many hugely talented women working in Britain today. All artists have a difficult time after graduation, but for women the financial pressures and family commitments are amplified. Too many female artists end up having to give up."

Iwona Blazwick, director of the Whitechapel Gallery

"I have 50% women artists, and 50% male artists in the gallery, and they're as strong as each other. A lot of contemporary art is photography- and film-based, and it doesn't have that same weight of history as painting and sculpture. I think that's one reason why women artists are strong in this area. That's where the revolution is taking place."

Maureen Paley, director of the Maureen Paley Gallery

"What's really interesting is the number of women who are important in the art world in general. Just in London, Iwona Blazwick runs the Whitechapel Gallery and Julia Peyton-Jones runs the Serpentine. I don't think it's entirely true women are being paid less: Dana Schutz is the most talked-about young artist in the US at the moment; she's only 32, but her paintings go for a quarter of a million dollars each."